Project Summary/Abstract Bilingualism is the norm for much of the world, and even in the United States, where monolingualism is common, at least 22% of individuals over age 5 speak a language other than English at home. However, we still do not know how to best structure the language environment to yield optimal language outcomes in bilingual children. In the current proposal, we aim to answer this fundamental question by examining the effect of dual language input on word learning in Spanish-English bilingual children with fluctuating levels of language ability. Specific Aim 1 is to examine the effect of dual language input on novel word learning. In these experiments we will contrast learning from single-language (English only) vs. dual language exposure (Spanish and English) across different types of dual-language input and for different word types. Specific Aim 2 is to examine the effect of dual language input on novel word consolidation. In these experiments, we will use eye-tracking to test whether newly-learned novel words enter into interactive dynamics with known words within and across languages. Specific Aim 3 is to examine the relationship between language ability and novel word learning under dual- language conditions. We will do so by recruiting bilingual children who occupy the full range of language abilities - from typical skills in both English and Spanish to weak skills in both languages (i.e., language impairment). This strategy will enable us to treat language ability as a continuum and to examine how it predicts novel word learning and consolidation across all experiments proposed under Aims 1 and 2. We hypothesize that dual-language input may be particularly challenging for children with weak language skills, and expect that the relationship between language ability and word-learning outcomes will vary across different types of dual-language input and different word types. Ultimately, we will be able to state whether children with weak language skills struggle more with learning from dual-language input than children with strong language skills, yielding practical recommendations for parents, clinicians, and educators. Together, our findings will have clear-cut practical consequences and far-reaching theoretical ramifications. Currently, advice to parents raising bilingual children with language impairment regarding the optimal language exposure is based on intuition rather than on solid empirical evidence. The current proposal represents the first step towards establishing empirically-grounded guidelines regarding optimal language input for bilingual children with and without language impairment. At the theoretical level, the study of code-switching has been central to the field of bilingualism, and we know a great deal about the process by which bilinguals switch languages. However, we know very little about how the different ways of switching languages influence children?s ability to learn. By focusing on the learning process itself, the proposed project can contribute vital information to the code-switching literature, while at the same time shedding light on how dual language input may shape lexical learning.